Gain control in nonlinear fiber amplifier stages

ABSTRACT

An apparatus and method are described for exploiting almost the full almost 25 TH % of bandwidth available in the low-loss window in optical fibers (from 1430 nm-1620 nm) using a parallel combination of optical amplifiers. The low-loss window at about 1530 nm-1620 nm can be amplified using erbium-doped fiber amplifiers (EDFAs). However, due to the inherent absorption of the erbium at shorter wavelengths, EDFAs cannot be used below about 1525 nm without a significant degradation in performance. For the low-loss window at approximately 1430-1530 nm, amplifiers based on nonlinear polarization in optical fibers can be used effectively. A broadband nonlinear polarization amplifier (NLPA) is disclosed which combines cascaded Raman amplification with parametric amplification or four-wave mixing. In particular, one of the intermediate cascade Raman order wavelengths λ r  should lie in close proximity to the zero-dispersion wavelength λ 0  of the amplifying fiber. For this intermediate Raman order, spectral broadening will occur due to phase-match 15 with four-wave mixing (if λ r &lt;λ 0 ) or phase-matched parametric amplification (if λ r &lt;λ 0 ). In further cascaded Raman orders, the gain spectrum will continue to broaden due to the convolution of the gain spectrum with the spectrum from the previous Raman order.

CROSS-REFERENCE TO RELATED CASES

This application is a continuation of U.S. patent application Ser. No.09/866,497, filed May 25, 2001 and entitled “NONLINEAR FIBER AMPLIFIERSUSED FOR A 1430-1530nm LOW-LOSS WINDOW IN OPTICAL FIBERS,” which is acontinuation of U.S. patent application Ser. No. 09/565,776, filed May5, 2000, now U.S. Pat. No. 6,239,902, which is a continuation of U.S.patent application Ser. No. 09/046,900 filed Mar. 24, 1998, now U.S.Pat. No. 6,101,024.

FIELD OF THE INVENTION

The present invention relates generally to optical amplifiers used infiber-optics for telecommunications, cable television and otherfiber-optics applications. More particularly, the invention relates toan optical fiber amplifier and method for producing an amplifiedbroadband output from an optical signal comprising a wavelength in therange of 1430-1530 nm.

BACKGROUND OF THE INVENTION

Because of the increase in data intensive applications, the demand forbandwidth in communications has been growing tremendously. In response,the installed capacity of telecommunication systems has been increasingby an order of magnitude every three to four years since the mid 1970s.Much of this capacity increase has been supplied by optical fibers thatprovide a four-order-of-magnitude bandwidth enhancement overtwisted-pair copper wires.

To exploit the bandwidth of optical fibers, two key technologies havebeen developed and used in the telecommunication industry: opticalamplifiers and wavelength-division multiplexing (WDM). Opticalamplifiers boost the signal strength and compensate for inherent fiberloss and other splitting and insertion losses. WDM enables differentwavelengths of light to carry different signals parallel over the sameoptical fiber. Although WDM is critical in that it allows utilization ofa major fraction of the fiber bandwidth, it would not be cost-effectivewithout optical amplifiers. In particular, a broadband optical amplifierthat permits simultaneous amplification of many WDM channels is a keyenabler for utilizing the full fiber bandwidth.

Silica-based optical fiber has its lowest loss window around 1550 nmwith approximately 25 THz of bandwidth between 1430 and 1620 nm. Forexample, FIG. 1 illustrates the loss profile of a 50 km optical fiber.In this wavelength region, erbium-doped fiber amplifiers (EDFAS) arewidely used. However, as indicated in FIG. 2, the absorption band of aEDFA nearly overlaps its the emission band. For wavelengths shorter thanabout 1525 nm, erbium-atoms in typical glasses will absorb more thanamplify. To broaden the gain spectra of EDFAs, various dopings have beenadded. For example, as shown in FIG. 3a, codoping of the silica corewith aluminum or phosphorus broadens the emission spectrum considerably.Nevertheless, as depicted in FIG. 3b, the absorption peak for thevarious glasses is still around 1530 nm.

Hence, broadening the bandwidth of EDFAs to accommodate a larger numberof WDM channels has become a subject of intense research. As an exampleof the state-of-the-art, Y. Sun et al. demonstrated in ElectronicsLetters, Vol. 33, No. 23, pp. 1965-67 (1997), a two-band architecturefor an ultra-wideband EDFA with a record optical bandwidth of 80 nm. Toobtain a low noise figure and high output power, the two bands share acommon first gain section and have distinct second gain sections. The 80nm bandwidth comes from one amplifier (so-called conventional band orC-band) from 1525.6 to 1562.5 nm and another amplifier (so-called longband or L-band) from 1569.4 to 1612.8 nm. As another example, M. Yamadaet al. reported in Electronics Letters, Vol. 33, No. 8, pp. 710-711(1997), a 54 nm gain bandwidth achieved with two EDFAs in a parallelconfiguration, i.e., one optimized for 1530-1560 nm and the otheroptimized for 1576-1600 nm. As yet another example, H. Masuda et al.reported in Electronics Letters, Vol. 33, No. 12, pp. 1070-72 (1997), a52 nm EDFA that used two-stage EDFAs with an intermediate equalizer.

These recent developments illustrate several points in the search forbroader bandwidth amplifiers for the low-loss window in optical fibers.First, bandwidth in excess of 40-50 nm require the use of parallelcombination of amplifiers even with EDFAS. Second, the 80 nm bandwidthachieved by Y. Sun et al., may be very close to the theoretical maximum.The short wavelength side at about 1525 nm is limited by the inherentabsorption in erbium, and long wavelength side is limited bybend-induced losses in standard fibers at above 1620 nm. Therefore, evenwith these recent advances, half of the bandwidth of the low-losswindow, i.e., 1430-1530 nm, remains without an optical amplifier.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical amplifierfor a range of wavelengths between about 1430 nm and 1530 nm.

It is another object of the present invention to provide a broadbandnonlinear polarization amplifier (NLPA) for generating a broadbandoutput from an optical signal having a wavelength between about 1430 nmand 1530 nm.

It is yet another object of the present invention to provide a paralleloptical amplification apparatus having a combination of the NLPA andEDFA for the almost full 25 THz bandwidth between 1430 nm and 1620 nm inthe low-loss window of optical fibers.

In accordance with the invention, a broadband NLPA is implemented byusing a combination of cascaded Raman amplification and eitherparametric amplification (PA) or four-wave mixing (4WM) in opticalfibers. To achieve the broad bandwidth, one intermediate order of theRaman cascade is arranged to be at a close proximity to thezero-dispersion wavelength of an amplifying fiber. This intermediateorder phase matches PA (if its wavelength is greater than thezero-dispersion wavelength) or 4WM (if its wavelength is less than thezero-dispersion wavelength). PA/4WM generates sidebands and broaden thepump band. In subsequent Raman orders, the gain bandwidth is furtherbroadened due to the convolution of the Raman gain band with the pumpband. To produce an amplified broadband signal out of the NLPA of theinvention, the optical signal to be amplified must have a wavelengthgreater than the zero-dispersion wavelength, which in turn must begreater than the pumping wavelength from a pumping means of the NLPA.

In one embodiment, a broadband NLPA employs a 1240 nm pump and anopen-loop fiber with a zero-dispersion wavelength corresponding to oneof the Raman orders (e.g., either 1310 nm or 1390 nm ordispersion-flattened in between). Another embodiment uses a Sagnac Ramancavity that is pumped at either 1117 nm or 1240 nm. Feedback by theSagnac Raman cavity reduces the required pump power, and the broadbandcavity design supports much of the generated bandwidth.

The present invention also relates to a parallel optical amplificationapparatus having a combination of optical amplifiers. In one embodiment,the parallel optical amplification apparatus comprises two parallelstages of NLPAs with one NLPA optimized for 1430-1480 nm and the otherfor 1480-1530 nm. In another embodiment, the full 25 THz of the low-losswindow of approximately 1430 nm to 1620 nm in optical fibers isexploited by using a parallel combination of a NLPA of the invention anda EDFA.

NLPAs have the advantage that the gain band is set by the pumpingwavelengths, and gain can be provided over virtually the entiretransparency region in optical fibers (i.e., between 300 nm and 2000nm). Moreover, because NLPAs utilize inherent properties of glassfibers, NLPAs can be used even in existing fibers by modifying theterminal ends. Hence, NLPAs are fully compatible with fiber-opticsystems and can take advantage of the mature fiber-optic technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and elements of the present inventionwill be better understood from the following detailed description ofpreferred embodiments of the invention in which:

FIG. 1 depicts the loss profile of a 50 km fiber and the gain band of atypical EDFA.

FIG. 2 depicts absorption and gain spectra of an EDFA.

FIG. 3a depicts emission spectra of four EDFAs with different corecompositions.

FIG. 3b depicts absorption cross-section of erbium-doped glass ofdifferent compositions.

FIG. 4 depicts a measured Raman-gain spectrum for fused silica at a pumpwavelength of 1000 nm.

FIG. 5 plots power gain coefficient 2 g versus phase vector mismatch Δkfor parametric amplification.

FIG. 6 demonstrates basic concepts of the NLPA of the invention.

FIG. 7 illustrates the spectral broadening and gain expected from PA fora pump power of 1W and different separations between the pump andzero-dispersion wavelength.

FIG. 8 illustrates the spectral broadening and gain expected from PA fora pump and zero-dispersion wavelength separation of 1 nm and for varyingpump powers.

FIG. 9 is a schematic illustration of a first embodiment of an NLPAusing an open-loop configuration, and FIGS. 9a-c show the three choicesof fibers.

FIG. 10 is a schematic illustration of a second embodiment of an NLPAusing a Sagnac Raman cavity that is pumped at 1240 nm.

FIG. 11 is a schematic illustration of a third embodiment of an NLPAusing a Sagnac Raman cavity that is pumped at 1117 nm.

FIG. 12 is a schematic illustration of a first embodiment of a paralleloptical amplification apparatus having two stages of NLPAs.

FIG. 13 is a schematic illustration of a second embodiment of a paralleloptical amplification apparatus that is a combination of an EDFA and anNLPA.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a structure for exploiting almost thefull 25 THz of bandwidth available in the low-loss window of opticalfibers from 1430 nm to 1620 nm. The broadband NLPA amplifier of theinvention combines Raman amplification with either PA or 4WM to achievebandwidth performance improvements that neither technology by itself hasheretofore been able to deliver.

More specifically, the broadband NLPA of the invention comprises aninput port for inputting an optical signal having a wavelength λ, adistributed gain medium for receiving the optical signal and amplifyingand spectrally broadening the same therein through nonlinearpolarization, a pumping means operated at wavelength λ_(p) forgenerating a pumping light to pump the distributed gain medium, and anoutput port for outputting the amplified and spectrally broadenedoptical signal. The distributed gain medium has zero-dispersion atwavelength λ₀ such that λ≧λ₀≧λ_(p). The pumping light cascades throughthe distributed gain medium a plurality of Raman orders including anintermediate order having a wavelength λ_(r) at a close proximity to thezero-dispersion wavelength λ₀ phase match four-wave mixing (if λ_(r)<λ₀)or parametric amplification (if λ_(r)>λ₀).

A first embodiment of the NLPA uses open-loop amplification with anoptical fiber gain medium. A pump source operated at 1240 nm is used.The pump may be retro-reflected to increase the conversion efficiency. Asecond embodiment of the NLPA uses a Sagnac Raman cavity that is pumpedat 1240 nm. Feedback in the Sagnac Raman cavity reduces the requiredpump power, and the broadband cavity design supports much of thegenerated bandwidth. A third embodiment of the NLPA uses a Sagnac Ramancavity pumped at 1117 nm for a very broadband operation.

The present invention also relates to a parallel optical amplificationapparatus having a combination of optical amplifiers. In one embodiment,the parallel optical amplification apparatus comprises two parallelstages of NLPAs with one NLPA optimized for 1430 to 1480 nm and theother for 1480 to 1530 nm. In another embodiment, the full 25 THz of thelow-loss window in optical fibers is exploited by using a parallelcombination of a Raman amplifier and a rare earth doped amplifier.Preferably, an NLPA of the invention is used cover the low-loss windowof approximately 1430 nm to 1530 nm, and an EDFA is used to cover thelow-loss window of approximately 1530 nm to 1620 nm.

To provide a better understanding of the amplification mechanisms atwork in the present invention, we first describe stimulated Ramanscattering, Raman cascading, PA and 4WM. Stimulated Raman scatteringeffect, PA and 4WM are the result of third-order nonlinearities thatoccur when a dielectric material such as an optical fiber is exposed tointense light. The third-order nonlinear effect is proportional to theinstantaneous light intensity.

1. Stimulated Raman Scattering

Stimulated Raman scattering is an important nonlinear process that turnsoptical fibers into amplifiers and tunable lasers. Raman gain resultsfrom the interaction of intense light with optical phonons in silicafibers, and Raman effect leads to a transfer of energy from one opticalbeam (the pump) to another optical beam (the signal). The signal isdownshifted in frequency (or upshifted in wavelength) by an amountdetermined by vibrational modes of silica fibers. The Raman gaincoefficient g_(r) for the silica fibers is shown in FIG. 4. Notably, theRaman gain g_(r) extends over a large frequency range (up to 40 THz)with a broad peak centered at 13.2 THz (corresponding to a wavelength of440 cm⁻¹). This behavior over the large frequency range is due to theamorphous nature of the silica glass and enables the Raman effect to beused in broadband amplifiers. The Raman gain also depends on thecomposition of the fiber core and can vary with different dopantconcentrations.

Raman amplification has some attractive features. First, Raman gain is agood candidate for upgrading existing fiber optic links because it isbased on the interaction of pump light with optical phonons in theexisting fibers. Second, there is no excessive loss in the absence ofpump power—an important consideration for system reliability.

2. Raman Cascading

Cascading is the mechanism by which optical energy at the pumpwavelength is transferred, through a series of nonlinear polarizations,to an optical signal at a longer wavelength. Each nonlinear polarizationof the dielectric produces a molecular vibrational state correspondingto a wavelength that is offset from the wavelength of the light thatproduced the stimulation. The nonlinear polarization effect isdistributed throughout the dielectric, resulting in a cascading seriesof wavelength shifts as energy at one wavelength excites a vibrationalmode that produces light at a longer wavelength. This process cancascade through numerous orders. Because the Raman gain profile has apeak centered at 13.2 THz in silica fibers, one Raman order can bearranged to be separated from the previous order by 13.2 THz.

Cascading makes stimulated Raman scattering amplifiers very desirable.Raman amplification itself can be used to amplify multiple wavelengths(as in wavelength division multiplexing) or short optical pulses becausethe gain spectrum is very broad (a bandwidth of greater than 5 THzaround the peak at 13.2 THz). Moreover, cascading enables Ramanamplification over a wide range of different wavelengths. By varying thepump wavelength or by using cascaded orders of Raman gain, the gain canbe provided over the entire telecommunications window between 1300 nmand 1600 nm.

3. Parametric Amplification and Four-Wave Mixing

PA/4WM involve two pump (P) photons that create Stokes (S) andanti-Stokes (A) photons. Both PA/4WM and Raman amplification arise fromthe third order susceptibility X⁽³⁾ in optical fibers. Morespecifically, the real part of X⁽³⁾ the so-called nonlinear index ofrefraction n₂, is responsible for PA/4WM, while the imaginary part ofX⁽³⁾ associated with molecular vibrations corresponds to the Raman gaineffect. In silica fibers, about ⅘ths of the n₂ is an electronic,instantaneous nonlinearity caused by ultraviolet resonances, while about⅕th of n₂ arises from Raman-active vibrations, e.g., optical phonons(see further description in M. N. Islam, Ultrafast Fiber SwitchingDevices and Systems, Cambridge University Press, 1992). The imaginarypart of this latter contribution corresponds to the Raman gain spectrumof FIG. 4.

Whereas Raman amplification is attractive for providing optical gain,PA/4WM offers an efficient method to broaden the bandwidth of theoptical gain. PA/4WM has a much smaller frequency separation betweenpump and signal than Raman amplification, and the frequency differencemay depend on the pump intensity. Just as in Raman amplification, themain advantage of PA/4WM gain is that it is present in every fiber.However, unlike the Raman effect, both PA and 4WM requirephase-matching. 4WM is usually inefficient in long fibers due to therequirement for phase-matching. However, PA can act asself-phase-matched because the nonlinear index of refraction is used tophase match the pump and sidebands. This is particularly true whenoperating near the zero-dispersion wavelength in fibers. When 4WM and PAoccur near the zero-dispersion wavelength of a single-mode fiber,phase-matching becomes automatic in the fiber. In 4WM, sidebands aregenerated without gain when the pump wavelength falls in the normaldispersion regime (where the pumping wavelength is shorter than thezero-dispersion wavelength). PA is 4-photon amplification in which thenonlinear index of refraction is used to phase match the pump andsidebands. For PA the pump wavelength must lie in the anomalous groupvelocity regime (i.e., where the pumping wavelength is longer than thezero-dispersion wavelength) and proper phase matching requires that pumpand signal be co-propagating.

To illustrate the PA/4WM gain, consider the gain coefficient as derivedin R. H. Stolen and J. E. Bjorkholm, IEEE J. Quantum Elect., QE-18, 1062(1982): $\begin{matrix}{g = \sqrt{\left( {\gamma \quad P} \right)^{2} - \left\lbrack {\left( \frac{\Delta \quad k}{2} \right) + {\gamma \quad P}} \right\rbrack^{2}}} & (1)\end{matrix}$

The first term under the square root sign corresponds to the third ordernonlinearity that couples the pump photons to the sidebands. The secondterm corresponds to the phase mismatch between the waves and it consistsof two parts: one due to the wave-vector mismatch at the differentwavelengths and the other due to the increase in nonlinear index inducedby the pump. The nonlinearity parameter is defined as $\begin{matrix}{\gamma = {{\frac{\omega}{c}\frac{n_{2}}{A_{eff}}} = {\frac{2\pi}{\lambda}\frac{n_{2}}{A_{eff}}}}} & (2)\end{matrix}$

Also, assuming that we are operating near the zero-dispersionwavelength, the propagation constant can be expanded as $\begin{matrix}{\left. {{\Delta \quad k} = {{- {\frac{\lambda^{2}}{2\pi \quad c}\left\lbrack \frac{D}{\lambda} \right.}_{\lambda,}}\left( {\lambda_{p} - \lambda_{o}} \right)}} \right\rbrack \Omega^{2}} & (3)\end{matrix}$

where

Ω=ω_(p)−ω_(s)=ω_(a)−ω_(p).  (4)

When the pump wavelength falls in the normal dispersion regime, thenD<0,∂D/2λ>0, (λ_(p)−λ₀)<0, so that Δk>0. In this case, g is alwaysimaginary, and there is no gain during the sideband generation process.This corresponds to the case of 4WM. If operation is in the anomalousgroup velocity dispersion regime, then D>0,∂D/2λ>0, (λ_(p)−λ₀)>0, sothat Δk<0. This is the regime of PA, and the nonlinearity helps toreduce the phase mismatch (i.e., the two parts in the second term inEquation (1) are of opposite sign). There is gain for PA, and the gainis tunable with the pump power. As an example, the power gaincoefficient 2 g is plotted schematically in FIG. 5 for operation in theanomalous group velocity regime. The peak gain (g_(peak)=γP) occurs atΔk_(peak)=−2γP. The range over which the gain exists is given by0>Δk>−4γP. Thus, the peak gain is proportional to the pump power, andthe Δk range is determined by the pump power. Consequently, fromEquation (2) we see that the bandwidth can be increased by increasingthe pump power, increasing the nonlinear coefficient n₂ or decreasingthe effective area A_(eff). Alternately, for a given required frequencyrange over which gain is required, the pump requirements can be reducedby increasing the effective nonlinearity (n₂/A_(eff)).

4. Broadband NLPA by Combining Raman and Either PA or 4WM

This invention leads to broadband gain for cascaded Raman amplificationby arranging at least one intermediate Raman cascade order at a closeproximity to the zero-dispersion wavelength λ₀ (e.g., within ±5 nm ofλ₀, optimally within ±2 nm). Either 4WM (if λ_(r)<λ₀) or PA (ifλ_(r)>λ₀) will lead to spectral broadening of that particular Ramanorder. Then, in subsequent Raman orders the bandwidth will grow evenfurther. It is further advantageous if the cascade Raman wavelengthλ_(r) lies to the long wavelength side of λ₀ (i.e., in the anomalousdispersion regime), so that parametric amplification can occur.

The basic concept of the broadband NLPA is illustrated in FIG. 6.Starting from the pump wavelength λ_(p), cascaded Raman amplificationcan be used in the first few stages. This is only if the pump is morethan one Raman shift or 13.2 THz away from the zero-dispersionwavelength. (If it is desired to keep higher efficiency in these initialsteps, a narrow band cavity design can be used, such as designs based ongratings or wavelength selective couplers.)

The key to the invention of broadening the gain bandwidth is that one ofthe intermediate Raman cascade orders lies at a close proximity to thezero-dispersion wavelength λ₀. By operating close to λ₀, it almostautomatically phase-matches either 4WM or PA. In the subsequent cascadedRaman orders, the gain bandwidth may continue to broaden. This occursbecause the effective gain bandwidth of Raman is the convolution of thebandwidth of the pump (in this case, the previous Raman cascade order)with the Raman gain curve. In this regard, see discussion in R. H.Stolen, et al., “Development of the stimulated Raman spectrum insingle-mode silica fibers,” Journal of the Optical Society of America B,Vol. 1 (1984), pp. 652-57. Thus, the basic idea is to take advantage ofthe property of Raman amplification that the gain spectrum follows thepump spectrum. As the pump wavelength changes, the Raman gain changes aswell, separated by the distance of optical phonon energy which in silicafibers is an approximately 13.2 THz down-shift in frequency.

If the fiber is conventional so-called standard fiber, thenzero-dispersion wavelength λ₀ is about 1310 nm. On the other hand, fordispersion-shifted fiber the zero-dispersion wavelength λ₀ can shift tolonger wavelengths by adding waveguide dispersion. Alternately, adispersion-flattened fiber could be used for low dispersion values overone or more of the Raman cascade orders. The additional advantage ofusing dispersion-flattened fiber is that the dispersion slope is small,so the gain bandwidth will be even larger (c.f. Equations (1) and (3)).

The Raman gain spectrum can follow the pump spectrum only so long asthere is nothing in the Raman cavity to restrict the bandwidth of thesubsequent orders. Therefore, for these higher cascade orders Ramanlaser schemes, it is not desirable to use either gratings or wavelengthselective couplers. On the other hand, the broadband cavity design ofthe Sagnac Raman amplifier and laser lends itself naturally to increasedbandwidth by tailoring the pump spectrum. A single-pass fiber designconstitutes the broadest bandwidth design. However, a broadband cavitysuch as the Sagnac Raman cavity is advantageous because the feedback isused to lower the threshold and the required pump power. Also, it shouldbe noted that broadening the bandwidth leads to a drop in efficiency, sothe pump powers are already higher for the broadband cavity designs.

5. Example of NLPA Gain Broadening from PA and Cascaded Raman

To illustrate the cascaded Raman amplification to reach the 1430-1530 nmrange of the low-loss window, consider pumping with a commerciallyavailable cladding-pumped fiber laser, which operates around 1060 to1140 nm. The various Raman orders, each separated by 13.2 Thz from theprevious order, are set forth in Table 1.

TABLE 1 Various Raman orders when pumping between 1060 and 1140 nm(separation of 13.2 THZ between orders) Wavelength (nm) Δλ 1060.00 51.861111.86 57.19 1169.05 63.39 1232.44 70.66 1303.11 79.26 1382.37 89.531471.90 101.93 1573.82 117.09 1070.00 52.86 1122.86 58.36 1181.22 64.761245.98 72.27 1318.25 81.17 1399.42 91.82 1491.25 104.72 1595.97 120.541080.00 53.88 1133.88 59.54 1193.42 66.14 1259.56 73.90 1333.47 83.111416.58 94.16 1510.74 107.57 1618.32 124.07 1090.00 54.91 1144.91 60.741205.65 67.54 1273.19 75.56 1348.74 85.09 1433.83 96.55 1530.38 110.491640.87 127.69 1100.00 55.95 1155.95 61.94 1217.89 68.96 1286.85 77.241364.09 87.10 1451.19 98.98 1550.17 113.47 1663.64 131.40 1110.00 57.001167.00 63.17 1230.16 70.40 1300.56 78.94 1379.50 89.14 1468.64 101.461570.10 116.52 1686.62 135.20 1117.00 57.74 1174.74 64.03 1238.77 71.411310.18 80.15 1390.33 90.59 1480.92 103.22 1584.15 118.69 1702.84 137.921120.00 58.05 1178.05 64.40 1242.46 71.85 1314.31 80.67 1394.98 91.221486.20 103.99 1590.19 119.63 1709.82 139.10 1130.00 59.12 1189.12 65.651254.77 73.32 1328.10 82.43 1410.53 93.33 1503.86 106.56 1610.42 122.811733.24 143.09 1140.00 60.20 1200.20 66.92 1267.12 74.82 1341.93 84.211426.14 95.48 1521.62 109.18 1630.81 126.07 1756.87 147.19

To obtain gain between 1430 nm and 1520 nm, the pump is operated between1090 nm and 1140 nm, and five cascaded Raman orders are used to reachthe desired wavelength. To make use of the broadening from PA or 4WM, apumping scheme is selected in the middle of this range, i.e., startingwith a pump wavelength of 1117 nm. Then, the various Raman orders landat approximately 1175 nm, 1240 nm, 1310 nm, 1390 nm and finally 1480 nm.In particular, the third Raman frequency (1310 nm) passes through thezero-dispersion point of a standard fiber, and the next order (1390 nm)could be close if the fiber is dispersion shifted. Therefore, abroadband gain is expected for wavelengths in the 1430-1530 nm rangecentered around 1480 nm by using a fiber with a standard dispersion anda pump wavelength of 1117 nm, 1175 nm or 1240 nm.

Next, consider the broadening expected from PA. Assume that a standardfiber is used and the pump wavelength starts at 1117 nm. Thecalculations use Equations (1-4) with the following typical parametersfor high-Raman cross-section fiber: λ₀=1310 nm,γ=9.9 W⁻¹km⁻¹, and adispersion slope of 0.05 ps/nm-km. In FIG. 7, the gain coefficient forPA is plotted versus wavelength at a pump power of 1W and wavelengthseparations λ_(r)−λ₀) of 0.5, 1, 2 and 5 nm. For a wavelength separationof 2 nm, the PA peak gain occurs at .±10 nm, so the spectral broadeningis over 20 nm. The closer the pump wavelength approaches thezero-dispersion wavelength, the wider the gain bandwidth. In addition,FIG. 8 plots the gain versus wavelength for a separation of (λ_(r)−λ₀)=1nm and pump powers of 0.7, 1, 2, and 3W. The peak gain increasesdirectly proportional to the pump power, while the bandwidth increasesas the square root of pump power.

6. Preferred Embodiments of NLPA Amplifiers

FIG. 9 shows a first embodiment of the invention which uses an open-loopdesign to produce an amplified broadband signal for a range ofwavelengths between 1430 nm and 1530 nm. The open-loop design is thesimplest nonlinear polarization amplifier although it may have a highpump power requirement. In the NLPA amplifier 20 as illustrated in FIG.9, an optical signal having a wavelength between 1430 nm and 1530 nm isinput from an input port 25 to an optical fiber 30. The optical fiber 30is pumped by a pumping light generated by a pumping laser 35 operated ata wavelength of about 1240 nm. The optical signal is amplified andspectrally broadened in the fiber by nonlinear polarization, and outputthrough an output port 40. The configuration is so arranged that theoptical signal has a wavelength greater than the zero-dispersionwavelength of the fiber, which in turn is greater than the pumpingwavelength of 1240 nm.

In this open-loop configuration, the fiber must have a cut-offwavelength below 1240 nm to be single-mode (spatial) over allwavelengths of the Raman cascade. Three choices of the fiber arepreferred in this configuration. First, a standard dispersion fiber witha zero-dispersion wavelength at about 1310 nm. Second, two fibersspliced together with one fiber having a zero-dispersion wavelength atabout 1310 nm (first cascade) and the other at 1390 nm (second cascade).Third, a dispersion-flattened fiber with low-dispersion at least between1310 nm and 1390 nm. The reduced dispersion slope of such adispersion-flattened fiber increases significantly the bandwidth for PAor 4WM.

Exemplary 1240 nm pump lasers include: (a) an 1117 nm cladding-pumpedfiber laser followed by a coupler-based or grating-based Ramanoscillator cavity (with gratings for 1117 nm, 1175 nm and 1240 nm); (b)an optically-pumped semiconductor laser; or (c) a chromium-dopedforsterite laser. At one end of the fiber, a 1240 nm retro-reflector 45is preferably placed to increase pumping conversion efficiency. Theretro-reflector is preferably a dichroic mirror or a 1240 nm grating.The input and output ports are preferably WDM couplers, and isolatorsshould be used at the input and output ports to prevent from lasing dueto spurious feedback. It is desirous to use a counter-propagatinggeometry to average out noise fluctuations in this open-loopconfiguration. It is also possible to use a co-propagating geometry.

To reduce the pump power requirements, a broadband cavity such as theSagnac Raman cavity is used. FIG. 10 illustrates an embodiment of theNLPA that uses a Sagnac Raman cavity design with a 1240 nm pump.Referring to FIG. 10, the Sagnac Raman cavity of the NLPA 60 is formedby a broadband mirror 70 and a loop mirror comprising a Raman gain fiber65 and an optical coupler 90 connected thereto. The Sagnac Raman cavitydesign is described in a U.S. patent application Ser. No. 08/773,482,and this patent application is incorporated herein by reference. Anoptical signal having a wavelength between 1430 nm to 1530 nm is inputthrough an input port 75 to the Raman gain fiber 65. A pumping laser 80operated at a wavelength 1240 nm generates a pumping light that pumpsthe fiber 65 through a coupling means 85. The optical signal isamplified and spectrally broadened in the fiber by nonlinearpolarization, and output through an output port 95. The configuration isso arranged that the optical signal has a wavelength greater than thezero-dispersion wavelength of the fiber, which in turn is greater thanthe pumping wavelength of 1240 nm.

The Raman gain fiber has the same characteristics as described above forthe open-loop design. Similarly, the pumping lasers used in the firstembodiment are used in this second embodiment. The broadband NLPA mayfurther include a polarization controller 100 in the Sagnac Raman cavityfor controlling polarization state. However, if the fiber ispolarization maintained, the polarization controller is not necessary.The optical coupler 90 is nominally 50:50 at least for the opticalsignal having a wavelength between about 1240 nm and 1430 nm. Thecoupling means 85 is a WDM coupler which transmits at least at awavelength between about 1300 nm and 1430 nm. Moreover, the input portand output port each comprises a WDM coupler which transmits at least ata wavelength between about 1240 nm and 1425 nm. A key advantage of theSagnac Raman cavity is that it has a passive noise dampening propertythat leads to quieter cascading of various Raman orders.

As an alternative, the same kind of Sagnac Raman cavity can be used forall five Raman cascade orders between 1117 nm and the low-loss window.FIG. 11 illustrates a third embodiment of a five-order Sagnac Ramanamplifier for NLPA operation. A cladding-pumped fiber laser operatingaround 1117 nm is used as a pumping laser 120. Different fibercombinations described in the first embodiment can be used except thatthe fibers must have a cut-off wavelength below 1117 nm to accommodatesingle-mode operation for the pump. An optical coupler 130 is nominally50:50 at least for the optical signal having the wavelength betweenabout 1117 nm and 1430 nm. A coupling means 125 is a WDM coupler whichtransmits at least at wavelengths between about 1165 nm and 1430 nm.Moreover, the input and output ports each comprises a WDM coupler whichtransmits at least at wavelengths between about 1117 nm and 1425 nm.Although the wavelength range of the various components increases, thisconfiguration leads to an even broader gain band since the pumpbandwidth is allowed to increase even during the first two cascadesbetween 1117 nm and 1240 nm. Also, the noise dampening property of theSagnac cavity can be used advantageously over all five Raman orders.

In general, the NLPA is operated as follows. An optical signal having awavelength λ is input through an input port into a distributed gainmedium having zero-dispersion at a wavelength λ₀, such as an opticalfiber, which is pumped by a pumping light from a pumping means operatedat a wavelength λ_(p), wherein λ≧λ₀>λ_(p). The pumping light cascadesthrough the distributed gain medium a plurality of Raman ordersincluding an intermediate order having a wavelength λ_(r) at a closeproximity to the zero-dispersion wavelength λ₀ to phase match four-wavemixing (if λ_(r)<λ₀) or parametric amplification (if λ_(r)>λ₀). Theamplified and spectrally broadened optical signal is output through anoutput port.

7. Broadband Parallel Optical Amplification Apparatus

The above embodiments demonstrate that a single NLPA can accommodate thefull bandwidth of the low-loss window. Moreover, the full bandwidth ofthe low-loss window may be reached by using a parallel opticalamplification apparatus having a combination of two or more Ramanamplifiers and rare earth doped amplifiers. Preferably, the NLPAs andEDFAs are used.

FIG. 12 shows a first embodiment of the parallel optical amplificationapparatus using a combination of two NLPAs for a range of wavelengthsbetween 1430 nm and 1530 nm. Referring to FIG. 12, a dividing means 170divides an optical signal having a wavelength between 1430 nm to 1530 nmat a predetermined wavelength, preferably at 1480 nm, into a first beamhaving a wavelength less than the predetermined wavelength and a secondbeam having a wavelength greater than the predetermined wavelength. Thefirst beam is input into a first NLPA 180 for amplification and spectralbroadening therein. The second beam is input into a second NLPA 190 foramplification and spectral broadening therein. Outputs from the firstand second NLPAs are combined by a combining means 200 to produce anamplified and spectrally broadened optical signal. The input port 170and output port 200 are preferably WDM couplers.

It is preferred that the first NLPA 180 is optimized for 1430-1480 nmand centered at 1455 nm, while the second NLPA is optimized for1480-1530 nm and centered at 1505 nm. From Table 1, these two windowscan be achieved in a five-order cascade by starting with a pumpwavelength of about 1100 nm for the short-wavelength side and a pumpwavelength of about 1130 nm for the long-wavelength side. For theshort-wavelength side, the fiber should have a zero-dispersion around1365 nm, while for the long-wavelength side, the fiber zero-dispersionshould be around 1328 nm or 1410 nm.

The narrower-bandwidth for each NLPA will also lead to an increasedefficiency for each amplifier. Furthermore, the components may be moreeasily manufactured, since the wavelength window is not as large.Finally, the multiple amplifiers may allow for gradual upgrades ofsystems, adding bandwidth to the EDFA window as needed.

A spectrum of 1430-1620 nm in the low-loss window is amplified andspectrally broadened by using a parallel optical amplification apparatuscomprising Raman amplifiers and rare earth doped amplifiers. FIG. 13describes a second embodiment of the parallel optical amplificationapparatus. The amplification apparatus comprises a broadband NLPA 240 asdisclosed in this invention and a EDFA 250. A dividing means 230 of theapparatus divides an optical signal having a wavelength between 1430 nmand 1620 nm at a predetermined wavelength, preferably at 1525 nm, into afirst beam having a wavelength less than the predetermined wavelengthand a second beam having a wavelength greater than the predeterminedwavelength. The broadband NLPA 240 receives the first beam and producesan amplified broadband first beam. The EDFA 250 receives the second beamand produces an amplified broadband second beam. A combining means 260combines the amplified and spectrally broadened first and second beamsto produce an amplified broadband optical signal. It is preferred thatboth the dividing means 230 and the combining means 260 are WDMcouplers.

To use any of these embodiments with multi-wavelength WDM channels, itwill most likely be necessary to include at the output of the amplifiersome means for equalizing the gain. This wavelength dependency ornonuniformity of the gain band has little impact on single-channeltransmission. However, it renders the amplifier unsuitable formultichannel operation through a cascade of amplifiers. As channels atdifferent wavelengths propagate through a chain of amplifiers, theyaccumulate increasing discrepancies between them in terms of gain andsignal-to-noise ratio. Using gain-flattening elements can significantlyincrease the usable bandwidth of a long chain of amplifiers. Forexample, the NLPA can be followed by a gain flattening element toprovide gain equalization for different channels. Alternately, the gainflattening element could be introduced directly into the Sagnacinterferometer loop of FIG. 10 or 11.

It is understood that various other modifications will be readilyapparent to those skilled in the art without departing from the scopeand spirit of the invention. Accordingly, it is not intended that thescope of the claims appended hereto be limited to the description setforth herein, but rather that the claims be construed as encompassingall the features of the patentable novelty that reside in the presentinvention, including all features that would be treated as equivalentsthereof by those skilled in the art to which this invention pertains.

What is claimed is:
 1. A nonlinear polarization amplifier stagecomprising: a gain medium operable to receive a multiple wavelengthoptical signal comprising wavelengths within a range λ_(s); a pumpoperable to generate at least one pump wavelength λ_(p) for introductionto the gain medium; and a coupler operable to introduce the at least onepump wavelength to the gain medium to facilitate Raman amplification ofat least a portion of the multiple wavelength optical signal; wherein atleast an intensity of the at least one pump wavelength is selected toaffect the shape of a gain curve associated with the multiple wavelengthoptical signal in the amplifier stage; and wherein at least a portion ofthe gain medium comprising a zero-dispersion wavelength λ₀, which isless than or equal to at least one signal wavelength λ_(s) and greaterthan or equal to λ_(p).
 2. The nonlinear polarization amplifier stage ofclaim 1, wherein the gain medium comprises a distributed gain medium. 3.The nonlinear polarization amplifier stage of claim 1, wherein the gainmedium comprises an optical fiber.
 4. The nonlinear polarizationamplifier stage of claim 1, wherein the zero-dispersion wavelengthcomprises approximately 1310 nanometers.
 5. The nonlinear polarizationamplifier stage of claim 1, wherein the zero-dispersion wavelengthcomprises approximately 1550 nanometers.
 6. The nonlinear polarizationamplifier stage of claim 1, wherein the zero-dispersion wavelength iswithin a range of 1390 nanometers and 1540 nanometers.
 7. The nonlinearpolarization amplifier stage of claim 1, wherein the wavelength rangeλ_(s) resides within a wavelength range of 1430 to 1530 nanometers. 8.The nonlinear polarization amplifier stage of claim 1, wherein at leasta portion of the gain medium comprises a zero-dispersion wavelength λ₀,which is within thirty (30) nanometers of λ_(p).
 9. The nonlinearpolarization amplifier stage of claim 1, wherein the pump comprises partof a pump assembly operable to generate a plurality of pump wavelengthswithin a wavelength range λ_(p1)-λ_(pn).
 10. The nonlinear polarizationamplifier stage of claim 9, wherein the pump assembly comprises agrating based Raman oscillator pumped by a cladding-pumped fiber laser.11. The nonlinear polarization amplifier stage of claim 9, wherein atleast a portion of the gain medium comprises a zero-dispersionwavelength λ₀, which is less than or equal to at least one wavelengthλ_(s) and greater than or equal to at least one pump wavelength λ_(p) ofthe plurality of pump wavelengths.
 12. The nonlinear polarizationamplifier stage of claim 9, wherein at least a portion of the gainmedium comprises a zero-dispersion wavelength λ₀, which is within thirty(30) nanometers of at least one pump wavelength λ_(p) of the pluralityof pump wavelengths.
 13. The nonlinear polarization amplifier stage ofclaim 9, wherein the wavelengths or the intensities of at least some ofthe plurality of pump wavelengths are selected to affect the shape ofthe gain curve associated with the multiple wavelength optical signal inthe amplifier stage.
 14. The nonlinear polarization amplifier stage ofclaim 9, further comprising a controller operable to control wavelengthsof the pump signals output from the pump assembly to affect the shape ofthe amplifier gain curve.
 15. The nonlinear polarization amplifier stageof claim 9, wherein at least some of the plurality of pump wavelengthsdiffer by at least twenty (20) nanometers.
 16. The nonlinearpolarization amplifier stage of claim 9, wherein at least some of theplurality of pump wavelengths differ by seventy (70) nanometers or less.17. A method of controlling the shape of a gain curve of a nonlinearpolarization amplifier stage, comprising: receiving a multiplewavelength signal at a gain medium of a Raman amplifier stage, whereinthe wavelengths of the multiple wavelength signal are within awavelength range λ_(s); selecting at least an intensity of a pumpsignal, wherein the resulting pump signal comprises a wavelength λ_(p);and introducing the pump signal to the gain medium to facilitateamplification of at least a portion of the multiple wavelength signalover at least a portion of the gain medium; wherein the shape of a gaincurve for the amplifier stage is determined at least in part based onthe selection of the wavelength or intensity of the pump signal; andwherein at least a portion of the gain medium comprises azero-dispersion wavelength λ₀, which is less than or equal to at leastone wavelength λ_(s) and greater than or equal to λ_(p).
 18. The methodof claim 17, wherein the gain medium comprises a distributed gainmedium.
 19. The method of claim 18, wherein the distributed gain mediumprovides distributed Raman amplification.
 20. The method of claim 17,wherein the zero-dispersion wavelength comprises approximately 1310nanometers.
 21. The method of claim 17, wherein the zero-dispersionwavelength comprises approximately 1550 nanometers.
 22. The method ofclaim 17, wherein the zero-dispersion wavelength is within a range of1390 nanometers and 1540 nanometers.
 23. The method of claim 17, whereinthe wavelength range λ_(s) comprises 1430 to 1530 nanometers.
 24. Themethod of claim 17, wherein at least a portion of the gain mediumcomprises a zero-dispersion wavelength λ₀, which is within thirty (30)nanometers of λ_(p).
 25. The method of claim 17, wherein the pump signalcomprises a multiple wavelength pump signal comprising a plurality ofpump wavelengths within a wavelength range λ_(p1)-λ_(pn).
 26. The methodof claim 25, wherein selecting at least an intensity of the pump signalcomprises selecting at least an intensity of at least some of aplurality of pump wavelengths.
 27. The method of claim 25, wherein theintensities of at least some of the plurality of pump wavelengths areselected to affect the shape of the gain curve associated with themultiple wavelength optical signal in the amplifier stage.
 28. Themethod of claim 25, further comprising changing the wavelength of a pumpsignal communicated to the gain medium to affect the shape of the gaincurve associated with the multiple wavelength optical signal in theamplifier stage.
 29. The method of claim 25, wherein at least some ofthe plurality of pump wavelengths differ by at least twenty (20)nanometers.
 30. The method of claim 25, wherein at least some of theplurality of pump wavelengths differ by seventy (70) nanometers or less.31. The method of claim 17, wherein at least a portion of the gainmedium comprises a zero-dispersion wavelength λ₀, which is within thirty(30) nanometers of at least one pump wavelength λ_(p) of the pluralityof pump wavelengths.
 32. A nonlinear polarization amplifier stagecomprising: a gain medium operable to receive a multiple wavelengthoptical signal comprising wavelengths within a range λ_(s), at least aportion of the gain medium comprising a zero-dispersion wavelength λ₀; apump operable to generate at least one pump wavelength λ_(p) forintroduction to the gain medium, wherein the at least one pumpwavelength λ_(p) is within thirty (30) nanometers of the zero-dispersionwavelength λ₀; and a coupler operable to introduce the at least one pumpwavelength to the gain medium to facilitate Raman amplification of atleast a portion of the multiple wavelength optical signal; wherein atleast an intensity of the at least one pump wavelength is selected toaffect the shape of a gain curve associated with the multiple wavelengthoptical signal in the amplifier stage.
 33. A method of controlling theshape of a gain curve of a nonlinear polarization amplifier stage,comprising: receiving a multiple wavelength signal at a gain medium of aRaman amplifier stage, at least a portion of the gain medium comprisinga zero-dispersion wavelength λ₀, wherein the wavelengths of the multiplewavelength signal are within a wavelength range λ_(s); selecting atleast an intensity of a pump signal, wherein the resulting pump signalcomprises a wavelength λ_(p) and wherein the wavelength λ_(p) is withinthirty (30) nanometers of the zero-dispersion wavelength λ₀; andintroducing the pump signal to the gain medium to facilitateamplification of at least a portion of the multiple wavelength signalover at least a portion of the gain medium; wherein the shape of a gaincurve for the amplifier stage is determined at least in part based onthe selection of the wavelength or intensity of the pump signal.